Improvement Of The Cathode Materials For Lithium Ion Battery Using Carbon Materials

Internal resistance is one of the key point of influencing the rate,cycling and safety of lithium ion batteries.In this thesis,we use several approaches to improve the conductivity of cathode material using carbon material,thus to improve the rate and cycling performance of cathode material of lithium ion batteries.Besides,we investigate the difference of cyclability of specific cathode material in half and full cell system.The effect of carbon sources and their mixing ratio on the rate,cycling and low temperature performance of Li FePO₄/C composite are investigated.LFP samples prepared with polyethylene glycol?PEG?and beta-cyclodextrin??-CD?as mixing carbon sources possesses the highest specific surface area and best rate capability,cycling stability and low-temperature discharge characteristic.CV and EIS results further indicate that PEG and?-CD as mixing carbon sources can decrease the charge transfer resistance and promote the3D electronic/ionic conductivities and Li-ion diffusion coefficients in the secondary structure,thus greatly improve the rate capability,cycling stability and low-temperature capacity of LFP cathode.A ferrocene-assisted calcination process?FACP?has been applied during the preparation of Li Fe_0.6Mn_0.4PO₄/C cathode material.The Li Fe_0.6Mn_0.4PO₄/C with FACP presents more uniformly and tightly coated carbon layers with higher graphitization degree than that of the sample without FACP assistance.The Li Fe_0.6Mn_0.4PO₄/C with FACP exhibits excellent cycling performance even at 60 ^oC.It delivers a capacity retention above96%after 100 cycles at 3 C,much better than that of the the sample without FACP.Using carbon nanotube?CNT?as electric conductivity,we investigate the key point influencing the dispersion of CNT in the cathode slurry.The results show that the content of functional group on CNT and the molecular weight of dispersant PVP are the key points.Cathode containing well dispersive CNT with higher electric conductivity shows lowest charge transfer resistance and best electrochemical performance.Cross-linked PAN coated LiCoO₂ was prepared through heat treatment in air,without causing surface damage of LiCoO₂.Its electrochemical performance was investigated and compared with that of bare and Al₂O₃ coated LiCoO₂.The results confirm that the cross-linked PAN is more effective in decreasing the side reaction between the cathode and electrolyte and depressing the growth of the charge transfer impedance of the electrode.Thus,the cross-linked PAN coating layer improves the cycling stability of Li CoO₂ under high cut-off voltage.We develop a one-step method to synthesize the W-coating NCM 523 secondary particles with uniform Li₂WO₄-coating layer.Its cycling stability in both half and full cell systems is investigated.It is found that,compared to the bare NCM 523,the W-coating NCM 523 show remarkable improvement in the capacity retention in half cells,but suffered more dramatic capacity decline in full cells.The reasons can be attributed to the side reaction with organic electrolyte and the dissolution of Li₂WO₄,which consumes the active lithium ions on the graphite anode during the cycling process.